supposed has 9 distinct synonyms spread across 2 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
based primarily on surmise rather than adequate evidence
“theories about the extinction of dinosaurs are still highly conjectural”
2 of the 4 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for supposed.
Synonyms, antonyms and the broader and narrower relations for supposed come from Princeton University’s WordNet. Full sources and licences.